The invention relates to nuclear medicine, and more particularly relates to nuclear medicine SPECT studies. In its most immediate sense, the invention relates to attenuation correction techniques for use in nuclear medicine SPECT studies in which fan beam collimators are employed.
In a conventional SPECT (Single Photon Emission Computed Tomography) study of e.g. the heart, a radioisotope (TC-99m, Tl-201, for example) is administered to the patient and the radioisotope is taken up by the heart muscles. Then, the patient is placed in a scintillation camera system and one or more scintillation camera detectors are rotated about the long axis of the patient. These detectors pick up gamma radiation which leaves the patient, and the resulting data is used to form three-dimensional images ("SPECT images" or "tomographic images") of the distribution of the radioisotope within the patient.
Such three-dimensional SPECT images can be calculated based on a set of two-dimensional images ("projections" or "projection images") acquired by the scintillation camera system; this calculation process is known as image reconstruction. The most commonly employed method of image reconstruction is known as "filtered backprojection". When filtered backprojection reconstruction is used to reconstruct SPECT images from scintigraphic projection images obtained from a scintillation camera, some well-known distortions introduce errors ("artifacts") in the result. One of the most important distortions is caused by attenuation of gamma radiation in tissue.
As a consequence of attenuation, image values in the various projections do not represent line integrals of the radioisotope distribution within the body. It is therefore necessary to correct for this, and the process for doing so in SPECT is known as attenuation correction.
Many techniques for attenuation correction in SPECT assume that the linear attenuation coefficient of the body is uniform and impose such uniformity as a mathematical constraint in the image reconstruction process. However, for a very important class of studies, namely cardiac SPECT studies, the linear attenuation coefficient of the body is in fact highly nonuniform. This is because lung tissue has a lower attenuation than do, e.g., the blood and other non-lung tissue.
Thus, in SPECT studies of, e.g., the heart, a SPECT reconstruction of the image of radioactivity within the heart will necessarily contain artifacts caused by the unequal attenuation coefficients of, e.g., the lungs and the body (and, in the case of some female patients, large breast size.)
It is known to measure the actual attenuation coefficients of body tissues by placing a source of gamma radiation on one side of the body and measuring the transmission of the gamma radiation through the body as a function of direction, i.e. collecting transmission CT data. When a fan beam collimator is used to carry out a cardiac SPECT study, it has been the practice to place a line source at the focus of the collimator. This insures proper registration between the CT data and the SPECT data and permits collection of the transmission CT data simultaneously with the SPECT data.
This practice has an undesirable consequence because of the nature of conventional fan beam collimators. Conventional fan beam collimators have the focal point located on the centerline of the collimator. Because of dimensional constraints imposed by the geometry of the camera gantry, use of a fan beam collimator and conventional filtered backprojection techniques causes truncation artifacts to be produced where the imaged region has a diameter of about 20 cm or more. Thus, where a cardiac SPECT study is to be carried out on a large or obese patient, the clinician cannot obtain the magnification advantages afforded by a fan beam collimator without also obtaining truncation artifacts in the reconstructed image.
It would therefore be advantageous to provide apparatus which would provide the magnification advantages of fan beam collimators on larger patients.
One object of the invention is to provide a fan beam collimator which can be used to image larger patients without having truncation artifacts in the reconstructed images.
Another object is, in general, to improve on known collimators and scintillation camera apparatus.
In accordance with the invention, the focus of a fan beam collimator is located elsewhere than on the centerline of the collimator. As a result, when such a collimator is used in a scintillation camera system, cardiac SPECT studies can be carried out on larger patients without truncation artifacts in the reconstructed image.